A. Introduction
The present invention relates to the drying of microspheres, and to the dry expansion of microspheres.
Microspheres are heat expandable thermoplastic polymeric hollow spheres containing a thermally activatable expanding agent. Such materials, the method of their manufacture, and considerable information concerning the properties and uses of microspheres, are all set forth in U.S. Pat. No. 3,615,972, issued to Donald S. Morehouse. Other teachings concerning such materials and uses are found in, for example, U.S. Pat. Nos. 3,864,181; 4,006,273; and 4,044,176.
In following the teachings of the Morehouse patent, the microspheres are made in an aqueous system by the limited coalescence process under pressure, and the resulting product is a "wet cake" of the unexpanded microsphere beads wetting agents and water. The wet cake is typically about 40 to 75 weight percent solids, and because of the wetting agents employed in the formation of the beads, the surface will be wet. Separation of water and beads has not been a simple process.
Many important uses of the microspheres require the removal of the water, to produce dry, free flowing microsphere beads. It is usual to pre-expand the beads before use. In unexpanded form, the dry microsphere beads typically have a displacement density of about 1.1 g/cm.sup.3. Dry, expanded microspheres typically have a density by displacement generally of less than 0.06 g/cm.sup.3, and are highly useful in the production of syntactic foams in a wide variety of polymer matrices. Dry, free-flowing, pre-expanded microspheres have now achieved a commercial recognition and market demand for such uses and others.
B. Prior Art
There have been to date few procedures by which dry expanded microspheres have been produced from wet cake. Dry free-flowing unexpanded microsphere beads have not heretofore been widely available except as a laboratory curiosity, and no commercially practicable procedure for producing such a product has as yet emerged, particularly because of the costs and limited quality of the products.
The techniques in the prior art for the production of expanded microspheres are limited in at least one of two aspects: either the product remains wet, or the expanded beads are produced in dry form with substantial amounts of agglomeration and a limited degree of expansion. The "acceptable" levels of agglomeration achieved in such procedures are in the range of about 3 to 10% of the product.
In U.S. Pat. No. 4,397,799, pre-expanded, dry, low density microspheres are produced by spray drying. The spray drying procedure has several disadvantages. First and foremost, the erection of a dedicated spray drying facility is required, representing a very substantial capital investment, and very considerable operating expenses, particularly for skilled labor and utilities costs in heating the drying fluid. In addition, the product is produced entrained in a heated, moving fluid stream, and the requirements of collection, recovery and handling are considerable. In addition, it is generally not feasible to produce the expanded product at the point of use, because of the size and expense of the facility, so that the spray drying approach largely mandates shipping of large volumes at considerable expense. Not to be neglected is the requirement that the spray drying be conducted under an inert atmosphere, since the usual blowing agents entrained within the microspheres are generally highly flammable, and often explosive. The usual procedure is to employ nitrogen as the spray drying fluid, with a necessary, and highly expensive safety burden on the system. Recovery of the expanded beads from the system is also demanding, not only to avoid product losses, but also because of the dust pollution of the working environment and atmosphere that can result. In addition, experience has shown that the spray drying technique is suited for developing expanded densities no less than about 0.032 to about 0.040 g/cm.sup.3 and typically about 0.036 g/cm.sup.3. Attempts at lower densities result in unacceptable levels of agglomeration, greater than about 10% of the product, and equally unacceptable product losses attributable to overexpansion, with attendant disruption of the bead structure.
Expansion of microsphere beads is attained in the process of U.S Pat. No. 4,513,106, where the wet cake is injected into a flow stream of steam, which is subsequently quenched in cool water. This produces pre-expansion of the microspheres, but still wet with water, at low solids content. While solids contents as high as fifteen percent have been occasionally been produced, the typical product is more often about 3 to 5 percent solids. The water limits the applications to which the procedure is applicable.
In our prior patent, U.S. Pat. No. 4,722,943, we disclosed a process wherein wet cake mixed with a processing aid, dried and expanded in an integral operation. In that invention, the processing aid is adhered to and embedded in the surface of the microspheres by thermal bonding, wherein the surface of the microspheres is heated to a temperature above the glass transition temperature, T.sub.g, so that the polymer material behaved as a hot melt adhesive to bind the processing aid. The equipment requirements for such operations are quite substantial, the procedure is difficult to control, and the quality and uniformity of the product are difficult to maintain. When carefully controlled, the product is within the limits heretofore considered "acceptable", with agglomeration levels in the range of 3 to 10%. While the procedure of our prior patent achieved a material reduction in capital costs and operating expenses to achieve acceptable product quality, it would be desirable to reduce costs still further and to achieve higher levels of quality and productivity. It would also be desirable to achieve greater expansion of the microspheres and to permit the use of lower proportions of the adherent surface barrier coating upon which the procedure is dependent. Experience has shown that when the microspheres are greater than 30% by weight of the mixture, the avoidance of acceptable levels of agglomeration becomes increasingly difficult with increasing proportions of microsphere beads. This has proved a serious problem, since it is product which is lowest in the proportion of the surface barrier coating and composite density which for the majority of uses is the more desired product.